Transparent conductors are widely used in the flat-panel display industry to form electrodes that are used to electrically switch the light-emitting or light-transmitting properties of a display pixel, for example in liquid crystal or organic light-emitting diode displays. Transparent conductors are also used to create electrodes for transparent capacitors used in capacitive touch-screen in conjunction with displays. In such displays, the transparency and conductivity of the transparent electrodes are important attributes. In general, it is desired that transparent electrodes have a high transparency (for example, greater than 90% in the visible spectrum) and a high conductivity (for example, less than 10 ohms/square).
Typical prior-art materials for such electrodes include indium tin oxide (ITO) and very thin layers of metal, for example silver or aluminum or metal alloys including silver or aluminum. These materials are coated, for example by sputtering or vapor deposition, and patterned on display substrates, such as glass. However, the current-carrying capacity of such electrodes is limited, thereby limiting the amount of power that can be supplied to the pixel elements. Moreover, the substrate materials are limited by the deposition process (e.g. sputtering). Thicker layers of metal oxides or metals increase conductivity but reduce the transparency of the electrodes.
Various methods of improving the conductivity of transparent conductors are taught in the prior art. For example, issued U.S. Pat. No. 6,812,637 entitled “OLED Display with Auxiliary Electrode” by Cok, describes an auxiliary electrode to improve the conductivity of the transparent electrode and enhance the current distribution. It is also known to provide wire grids on transparent substrates to provide optical control of incident light. For example, U.S. Pat. No. 6,532,111 describes a wire-grid polarizer. However, the formation of such metal grids is problematic. Sputtering through a shadow mask is difficult for large substrates due to thermal expansion and alignment problems of the shadow mask. Likewise, evaporative deposition of conductive materials such as metals requires high temperatures and suffers from the same mask problems. High temperatures can also destroy any temperature-sensitive underlying layers or substrates. The use of photolithography to pattern metal layers, metal-oxide layers, or metal grids can compromise the integrity of underlying layers. Furthermore, a metal grid is not transparent and can cover only a relatively small proportion of the transparent conductor area, reducing the conductivity of the auxiliary electrode.
It is also known in the prior art to form conductive traces using nano-particles comprising, for example silver. The synthesis of such metallic nano-crystals is known. For example, U.S. Pat. No. 6,645,444 B2 entitled “Metal nano-crystals and synthesis thereof” describes a process for forming metal nano-crystals optionally doped or alloyed with other metals. US20060057502 A1 entitled “Method of forming a conductive wiring pattern by laser irradiation and a conductive wiring pattern” describes fine wirings made by a method having the steps of painting a board with a metal dispersion colloid, drying the metal dispersion colloid into a metal-suspension film, irradiating the metal-suspension film with a laser beam of 300 nm-550 nm wavelengths, depicting arbitrary patterns on the film with the laser beam, aggregating metal nano-particles into larger conductive grains, washing the laser-irradiated film, eliminating non-irradiated metal nano-particles, and forming metallic wiring patterns built by the conductive grains on the board thus enabling an inexpensive apparatus to form fine arbitrary wiring patterns on boards without expensive photo-masks, resists, exposure apparatus and etching apparatus. US20060003262 similarly discloses a method of forming a pattern of electrical conductors on a substrate, wherein metal nano-particles can be mixed with a light-absorbing dye, and the mixture is then coated on the substrate. However, the wirings made with such materials are not transparent, particularly in combination with desired conductivity.
U.S. Pat. No. 4,394,661 relates to a thin metal masking film that will coalesce or “ball up” when heated rapidly with a high-intensity laser beam. This reduces the coverage of the metal film over a substrate and increases optical transmission. However, there is a problem with using such an element in that the optical density is not sufficient for many applications. If a thick metal film is employed in order to increase optical density, then the efficiency for coalescence decreases and the size of the debris created upon heating increases. U.S. Pat. No. 4,650,742 relates to a method of using an optical recording medium having two metal layers sandwiching a sublimable organic layer. There is a problem with this method, however, in that removing the sublimable organic layer requires a material collection apparatus and can be environmentally detrimental. U.S. Pat. No. 4,499,178 relates to a method of using an optical recording material where a heat insulating layer is interposed between a metallic recording layer and a reflecting layer. There is a problem with using this method in that the reflecting layer does not coalesce and therefore does not add to the image contrast. U.S. Pat. No. 6,243,127 describes a process of forming an image using a multi-layer metal coalescence thermal recording element. However, these prior-art methods do not form conductive and transparent electrodes.
As is known and practiced in the prior art, multiple layers of transparent conductors patterned on one or more transparent substrates can form capacitive arrays used in touch screens. In these applications, it is important to align the multiple layers to improve the capacitance of the layers of transparent conductors and to provide coverage of capacitors over the transparent substrate. Such alignment and patterning of multiple layers of transparent conductors is typically done with high-resolution photolithography equipment. Such equipment can be very expensive and limit manufacturing throughput and the materials used have limited conductivity and transparency.
There is a need, therefore, for an improved method for providing increased conductivity and transparency to the electrodes of a capacitive device that is scalable to large sizes, avoids heating materials in sensitive locations, enables simple layer alignment, and avoids the use of chemical processes and photolithographic equipment.